CHAP, xxv SYMBIOSIS OF PLANTS WITH ONE ANOTHER 87 



Perhaps we have here a notable example of one plant being aided by 

 another to settle in a habitat and to secure nutritive material on a soil 

 from which it would otherwise be excluded ; Calluna-heath, spruce- 

 forest, and the like, would then to a certain extent owe their existence 

 to such symbiosis. But a very great deal concerning this form of symbiosis 

 is completely unexplained. 1 



Apparently similar in some respects to endotrophic mycorhiza is 

 the symbiosis of Leguminosae and bacteria which has already been 

 mentioned on p. 80. The small root-tubercles of Leguminosae are 

 entered and occupied by bacteria which manufacture nitrogenous food 

 and finally perish, becoming changed into ' bacteroids ' and utilized as 

 food by the Leguminosae. It is not definitely established that the 

 bacteria profit by this symbiosis (they presumably acquire carbon- 

 compounds for their host) ; but if they did not profit it would be remark- 

 able that they, like endotrophic fungi, should enter roots. 



Going one step farther, we come to plants (algae) which inhabit 

 others without, so far as we know, doing any service in return. They 

 do not live at the expense of the host, in fact perhaps absorb nothing 

 whatever from it, but have free quarters. In this category may be 

 placed the cyanophyceous Anabaena living in the under-side of the leaves 

 of Azolla within special cavities ; these seem to exist only on the alga's 

 account, as they occur in all four species of Azolla which are never free 

 from Anabaena. The alga can flourish quite apart from Azolla. 



In like manner other algae live as endophytes, that is to say, inside 

 other plants : in Sphagnum, whose leaves are occupied by Nostoc, which 

 enters the colourless cells by way of the pores in their walls ; in certain 

 liverworts, or in algae for instance, Entoderma viride living in the cell- 

 wall of Derbesia Lamourouxii. But perhaps the last is an example of 

 parasitism. 



To some extent the same is presumably true in the case of those 

 Cyanophyceae which enter the erect dichotomous roots of cycads, and 

 stimulate a definite layer of parenchyma to grow in a special manner so as 

 to provide space for themselves ; also, in the case of Nostoc punctiforme, 

 which penetrates the stems of Gunnera but can live equally well apart 

 from roots or stems. 2 The present state of our knowledge does not permit 

 us to define the exact nature of the symbiosis in every case. 



EPIPHYTES. 



From those endophytes that only seek for accommodation in other 

 plants but do not absorb food from these, it is but a slight step to epiphytes, 

 or plants living on others but abstracting no food from the living parts 

 of the latter, and at most deriving sustenance from the dead tissue of 

 these. Still it is not always permissible to say that epiphytes do not live 

 at the expense of the supporting plants, for they may occur on these in 

 such quantities as to necessitate the assumption that they do injury 

 by this, very quantity, by causing excessive humidity, or by diminishing 

 respiration, as, for instance, in the case of lichens on trees. 



1 Those who have worked on mycorhiza include Kamiensky, 1881 ; Frank, 

 1887 ; Sorauer, 1893 ; Percy Groom, 1895 ; W. Magnus, 1900; Maze, 1809; Stahl, 

 1900 ; P. E. Muller, 1886, 1902, 1903 ; and many others. * B. Jonsson, 1894. 



